Porous cermet electrode and method of making same

Abstract

A method of making a highly porous cermet electrode on a solid electrolyte. The solid electrolyte is coated with noble metal and glass particles and heated to fuse the glass particles. The coated electrolyte is then annealed to form a separately leachable borate phase in the glass. That phase is then leached from the coating to leave a highly porous silica matrix throughout which the conductive particles are exposed.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A method of making a porous cermet electrode on a solid electrolyte substrate comprising: coating a surface of the substrate with a slurry of noble metal particles and glass particles, the glass being annealable into an immiscible and leachable borate phase;   heating the coated substrate at a first temperature to fuse the glass and bond it to the noble metal particles and to the substrate;   heating the coated substrate at a second and lower temperature for a sufficient duration to form an interconnected network of the borate phase leachable from a second glass phase within which it is dispersed; and   leaching the borate phase from the other glass phase, effective to produce a porous matrix throughout which said noble metal particles are exposed.   
     
     
       2. A method of making a porous cermet electrode on a solid electrolyte substrate comprising: coating a surface of the substrate with a slurry of noble metal particles and glass particles, the glass being annealable into a silica phase and an immiscible and leachable borate phase;   heating the coated substrate to a temperature of above about 900° C. to fuse the glass and bond it to the noble metal particles and to the substrate;   annealing the coated substrate after coating fusion at a temperature of about 500°-750° C. at least a few hours to form a leachable borate glass phase dispersed within a silica phase as a reticulated network; and   leaching the borate phase from the silica phase in said coating, effective to produce a highly porous silica matrix throughout which said noble metal particles are exposed.   
     
     
       3. A method of making a porous cermet exhaust electrode on a zirconia solid electrolyte body comprising: coating a surface of the body with a slurry of electron conductive particles selected from the group consisting of platinum particles, palladium particles, and particles of alloys thereof, a vehicle, and glass particles, the glass being annealable into a silica phase and an immiscible and leachable borate phase;   evaporating the vehicle from the coating;   heating the coated body to a temperature of at least 1000° C. to bond the coating to the body;   then heating the coated body at a temperature of about 600°-750° C. for at least about five hours to precipitate a borate phase and a silica phase, wherein the borate phase is in the form of interconnected channels extending between major faces of the coating; and   immersing the coated body in a mineral acid that selectively attacks the borate phase, whereby the borate phase is etched from the coating and a highly porous silica matrix is retained, throughout which the electron conductive particles are exposed.